Abstract : Spectrins are cytoskeletal proteins located at the inner face of the plasma membrane, making connections between membrane anchors and the actin cortex, and between actin filaments. Spectrins share a common structure forming a bundle of 3 a-helices and play a major role during cell deformation. Here, we used high-speed force spectroscopy and steered molecular dynamics simulations to understand the mechanical stability of spectrin revealing a molecular force buffering function. We find that spectrin acts as a soft spring at short extensions (70-100Å). Under continuous external stretching, its α-helices unwind leading to a viscous mechanical response over larger extensions (100-300Å), represented by a constant-force plateau in force/extension curves. This viscous force buffering emerges from a quasi-equilibrium competition between disruption and reformation of a-helical hydrogen bonds. Our results suggest